It is well known in the solenoid valve art that solenoid coil power is supplied by either a longitudinal regulator or a switching regulator. In either case, the function of the regulator is to adjust and control the supply voltage at an appropriate level for the magnetizing solenoid coil.
Since a switching regulator is generally more efficient than a longitudinal regulator, it is usually preferred in solenoid valve applications. A switching regulator alternately switches the supply voltage on and off to the solenoid coil. When the supply is switched on, the resultant current flow in the solenoid coil rises exponentially. When the current level reaches an upper limit value, the regulator shuts the supply off. As a result, the current then drops exponentially. When the current reaches a lower limit value, the supply voltage is again switched on by the action of the regulator. Thus, a mean supply voltage is established, which is regulated at a suitable value for a particular magnetizing solenoid coil. This type of process is disclosed in the German patent application DE 38 17 770.
To be able to switch on a solenoid valve rapidly and reliably, a relatively strong current is initially required for the magnetizing coil. The resulting magnetic force causes the armature of the solenoid to be moved against the force of a return spring. When the armature has reached its switch on position, the magnetizing current is lowered to a holding value, which is sufficient to hold the armature in place. This holding current is preferably adjusted to the lowest possible value, in order to save energy during continuous operation. There is, of course, a nominal holding current limit for reliably preventing the armature from dropping off. This nominal current limit must be set conservatively, i.e., at a higher than minimal level, if the solenoid valve is located in an area exposed to mechanical shock and vibration. This is generally the case, for example, if the solenoid valve is installed in a motor vehicle, especially in the proximity of the engine. This type of environment may cause a holding armature to drop off accidentally, causing the solenoid valve to shut off (or on) a pressure medium. Such an erroneous actuation should be avoided in any situation, but especially when the solenoid valve is used in an application that is relevant for safety, e.g., in the anti-lock braking system of a vehicle.
In the prior art, as disclosed in German patent applications DE 27 28 666 and DE 38 17 770, there are known techniques for recognizing the type of solenoid failure caused by a jammed armature. In these configurations, the switch on current waveform is monitored to detect a sudden deflection, which typically occurs at the time of armature movement.
However, the present invention has a different objective; namely, to detect armature drop off from a holding position as a result of a mechanical disturbance, and to take immediate corrective actions, including automatic restart and transmittal of an error message.
It is a further object of the present invention to detect armature drop off without the addition of special sensors.
It is yet a further object of the present invention to enable a significant reduction in the holding current safety margin, to such an extent that the power loss of a solenoid valve in continuous operation can be reduced by half. As a result, the structural volume of the solenoid valve can also be reduced.